Generally, a technology of decomposing a raw material gas by a glow discharge for forming a desired amorphous semiconductor film on a substrate, is used for forming an amorphous silicon film for a solar battery or an electrophotographic photoreceptor.
In manufacturing a solar battery, an electric power of high frequency is supplied to one parallel plate electrode of a pair of electrodes to cause a glow discharge between electrodes, whereby the other parallel plate electrode is grounded. Thus, an amorphous silicon film is formed on the surface of the substrate which is placed on the other parallel plate electrode. The method for forming a film on a substrate employing parallel plate electrodes is disclosed in, for example, U.S. Pat. No. 4,438,723, wherein a plurality of discharge electrodes are employed for causing a glow discharge. The electric power having the same frequency is separately supplied from a plurality of power supplies to each of the discharge electrodes. Although each power supply is assumed to have the same frequency, actually the frequencies applied to the discharge electrodes differ very slightly from each other. Therefore, the electromagnetic waves of high frequency from all discharge electrodes interfere with each other, causing a beat, a fluctuation, and so on. As a result, the discharge generated in the chamber becomes uneven. This presents a disadvantage in manufacturing because optimal conditions for forming an even film on the substrate can hardly be maintained.
A method for forming a film on an outer cylindrical wall surface of a cylindrical substrate, for example a method for forming an electrophotographic photoreceptor, is disclosed in, for example, U.S. Pat. No. 4,501,766, wherein a plurality of discharge electrodes are arranged opposite to each other to surround a plurality of substrates. The electric power of the same frequency is branched from a single power supply and applied to each of the plurality of the discharge electrodes. However, since the electric power of high frequency is supplied from a single power supply in such structure, the electric power of high frequency applied to each electrode cannot be separately controlled. Therefore, it is difficult to set the optimal conditions for forming a film of a uniform thickness. In addition, if the area of each discharge electrode differs from each other in the known structure, for example, if two cylindrical discharge electrodes comprising an inner wall and an outer wall are used, the power density of each electrode becomes uneven. In that case, it is difficult to adjust the state of each electrode to the same condition.
Therefore, it is difficult to make films having uniform characteristics so long as the film is formed by employing apparatuses of the above described conventional structures. For example, in manufacturing an electrophotographic photoreceptor, defects such as pin holes are generated in the film, so that the quality of the image cannot be enhanced In manufacturing a solar battery, the conversion efficiency cannot be enhanced.
Japanese Patent Publication Gazette No. 30130/1984 discloses a method for forming a film on a substrate by applying a plurality of different frequencies superimposed on each other to a single discharge electrode to cause a discharge.